U.S. Pat. No. 4,857,799 describes an example of a conventional colour field emission display. Such displays typically comprise a cathodoluminescent screen overlying and spaced from a two dimensional matrix of field emission cathodes. U.S. Pat. Nos. 3,789,471, 3,665,241, and 3,775,704 describe examples of, and methods of producing such cathodes. Each cathode comprises three arrays of field emissive tips. Each array covers a surface area of typically 300.times.300 microns. The arrays each comprise substantially the same number of tips (typically 1000). The screen is divided into a plurality of pixels. Each pixel is divided into three sub-pixels. Each sub-pixel is formed by a phosphor corresponding to a different one of the three primary colours, Red, Green and Blue. Each array of a cathode faces a different sub-pixel of a corresponding pixel. The arrays are individually addressable via row and column conductors.
In operation, voltages determined by red, green and blue input video signals are sequentially applied to the row and column conductors to address each cathode in turn in a raster fashion. The voltages interact to generate a localised high electric field at each tip. The localised electric fields cause electrons to be emitted from the tips by a tunnelling mechanism. The electrons are collectively accelerated towards the phosphors by an electric field generated between the screen and the cathode matrix. The phosphors are excited by incident electrons to display an image as a function of the input video signals.
In some conventional field emission displays, the screen is spaced from the cathodes by ball spacers of 200 micron diameter. The spacers are formed from a dielectric material. The dielectric material can be a plastics material such as polypropylene or an inorganic material such as glass. An organic adhesive secures the spacers in place. However, such adhesives tend to volatilise in evacuated environments such as the interior of the display. The volatilised adhesive contaminates the otherwise clean interior of the display. This can degrade the performance of the display. Furthermore, each spacer obscures a substantial portion of the underlying cathode.
In other conventional field emission displays, the screen is spaced from the cathodes by an array of pillars etched from a deposited film of polyimide of a thickness in the range 100 to 200 microns. The pillars permit displays with high aspect ratios to be fabricated. However, the pillars are again organic. Thus, as in the arrangement described above, the performance of the display can be degraded by the volatilisation of the pillar material. Furthermore, the fabrication of the pillars involves typically six or seven process steps that add expense and reduce yield.
In conventional gas plasma displays, spacers are deposited by thick film printing techniques. The spacers are deposited in the form of a ceramic-based ink which is then cured by firing. The disadvantage with this process is that gas plasma displays with spacer aspect ratios greater than 2:1 are difficult to achieve.